Premixed castable refractories (hereinafter referred to as “premixed materials”), which are obtained by tempering castable refractory powder compositions with water or other tempering liquids in advance in other sites than casting sites, such as factories, etc., and transported to the casting sites, have recently become widely used as refractories for linings of vessels for molten metals such as troughs for blast furnaces, ladles, tundishes, etc.
Though the premixed materials suffer from new problems such as hardening with time, the separation of aggregate or water during transportation, etc., they are advantageous over conventional castable refractory materials in (a) reduced unevenness in the properties of refractory bodies obtained therefrom because of a stabilized amount of tempering water and sufficient tempering, (b) the omission of a tempering step leading to reduced labor in site and the generation of no dust, resulting in improved working environment, and (c) no hardening even if a casting operation is once stopped, because the materials are not hardened at room temperature, etc. Because of these advantages, the premixed materials have become widely used.
However, attention has recently become paid to the hardening of the materials again. Because the premixed materials are not hardened at room temperature, it is necessary to harden the materials by some means to remove molds after casting. The premixed materials are hardened usually by heating via the molds. As such a heat-hardening method, JP 4-83764 A and JP 6-48845 A disclose methods for thermally hardening premixed materials containing heat-hardening agents at a temperature of 80° C. or higher. However, these heat-hardening methods are disadvantageous in high casting cost because of energy loss due to heat-hardening, and periodic maintenance due to the thermal deformation of molds, etc. They are also disadvantageous in the reduced strength of the resultant refractory body due to rapid water removal by heating.
To solve the problems of these heat-hardening methods, a method of adding a room-temperature-hardening binder to a premixed material immediately before casting, and hardening the material at room temperature was newly proposed. For instance, JP 5-60469 A uses alumina cement in the form of slurry as the room-temperature-hardening binder. However, because the alumina cement slurry is hardened with time, it cannot be produced in advance, but should be prepared at the time of casting.
On the other hand, JP 2000-16843 A proposes an alumina cement composition having an extremely long working time, its gunning method and an unshaped refractory obtained by such a gunning method. It can be stored and used days after gunning without discarding a castable refractory remaining in an apparatus because of a extremely long working time. Accordingly, the gunning method using such an alumina cement composition enables the reduction of cost and working.
The alumina cement composition of JP 2000-16843 A comprises alumina cement prepared from a clinker having a crystalline mineral composition comprising 60 to 95% by weight of CaO.2Al2O3, 5 to 30% by weight of 2CaO.Al2O3.SiO2 and 10% or less by weight of CaO.Al2O3, and a hardening retarder for the alumina cement. This reference discloses that it is preferable to use as a retarder at least one selected from the group consisting of phosphoric acids, a boric acid, silicofluorides, hydroxycarboxylic acids, polycarboxylic acids, polyhydroxycarboxylic acids, polyoxyalkylenes and saccharides. However, the alumina cement clinker mineral described in this reference comprises CaO.2Al2O3 and 2CaO.Al2O3.SiO2 as main components, its hydrating activity is extremely low. Accordingly, though hardening retarders composed of alkaline salts such as sodium tripolyphosphate, sodium citrate, sodium polyacrylate, etc., which are described in Examples of this reference, exhibit an effective retarding effect on the alumina cement based on low-hydrating-activity clinker minerals, their retarding effect on common alumina cement based on high-hydrating-activity CaO.Al2O3 is as insufficient as failing to reach 24 hours.